The invention relates to a fast automatic gain control arrangement provided in the intermediate frequency portion of a radio-frequency receiver receiving data from multiple transmission channels with frequent channel change. Such an arrangement includes an AGC amplifier which, when a channel is in use, is controlled by applying to it a control voltage whose digital value is taken from a memory in which this value was stored during the preceding use of the channel, and is also transferred to an adder for adding it algebraically to the digital value of a correction signal produced in a circuit for measuring and comparing the output voltage of the AGC amplifier in order to provide the digital value of the corrected control voltage.
In an amplitude-modulated transceiver set, a conventional AGC arrangement is formed by a phase-locked loop comprising a variable-gain amplifier which is included in the intermediate frequency portion of the receiver and a circuit for processing the control voltage obtained by detection of the signal at the output of the amplifier. A serious and often difficult problem is the optimization of the time constants of the activator and deactivator devices, the time constant of the activator device being the higher constant having a duration of some hundreds of milliseconds.
In certain transceiver apparatuses which are used particularly for data transmission, the transmission is effected in accordance with the high-rate multiple channel method. This is, for example, the case with frequency-deviation links which are usually designed for secret transmissions and multi-point link systems. Because the time interval relative to a frequency level or to an establishment of links with points where several participants are located can be very short, the comparatively long time constants of a conventional AGC arrangement are the main limitation of such systems.
A faster-responding automatic gain control arrangement, which takes these specific operating conditions into account and whose design is identical to the arrangement already set forth in the opening paragraph, is described in French Patent Specification No. 2,075,201. That arrangement relates to a transceiver by means of which variable-frequency data transmission is possible. The AGC amplifier is controlled by a memory which is formed by a matrix comprising a shift register which operates at the rate at which the frequency changes and whose number of memory locations is the same as the number of frequencies of the transmitted carrier. Knowing the previous state of the control voltage level stored in the memory and the fact that the frequency changes are utilized with such a recurrence that the variations of the signal attenuations between two consecutive uses of the same frequency are comparatively small renders it possible to reduce the response time of the arrangement. It should, however, be noted that the corrected value of the control voltage conveyed to the first register cannot be applied to the AGC amplifier until after it has arrived in the last register, that is to say at the end of a time interval equal to the repetition period of the successive frequencies.